1. Field of the Invention
The present invention relates to a photothermographic imaging material, and particularly to a photothermographic imaging material with high density which is excellent in light radiated image stability, silver color tone and the like, and to a method for forming an image by using the same.
2. Description of Related Art
In the fields of medical care and print plate making, waste solutions involved in wet processings of image formation materials have been problematic in terms of working property, and reduction of processing waste solutions has been strongly desired in the light of environmental preservation and saving space in late years. As a result, a photothermographic imaging material has been put to practical use and has rapidly become common.
A thermographic imaging material (hereinafter, simply referred to as a thermographic material or a photoconductive material) itself has proposed for long. For example, U.S. Pat. Nos. 3,152,904 and 3,457,07 are disclosed.
This photothermographic material is processed by a thermal development apparatus which adds stable heat to the photothermographic material to form the image, typically called a thermal developing apparatus. As mentioned above, in conjunction with the recent rapid prevalence, this thermal developing apparatus has been supplied in the market in large quantities. In the meanwhile, there has been problematic in that slipping property between the imaging material and a transport roller or processing members of the thermal developing apparatus changes, and transport failure and density unevenness occur. Also there has been problematic in that the density of the photothermographic imaging material varies with time. It has been found that these phenomena noticeably occur in the photothermographic imaging materials where image exposure is performed by laser light and subsequently the image is formed by thermal development.
Also recently, downsizing of laser imagers and acceleration of processings have been required. Therefore property improvement of the photothermographic imaging materials becomes essential.
For downsizing the thermal development processing apparatus, it is more advantageous to use a heat drum mode than to use a horizontal transport mode, but there has been problematic in that powder drop off, density unevenness and roller mark easily occur at the thermal development processing. Also, even when the rapid processing is carried out, to obtain sufficient density of the photothermographic imaging material, it is effective to enhance covering power by increasing coloring point numbers using silver halide with smaller average particle size as shown in JP Tokukaihei-11-295844A and JP Tokukaihei-11-352627A, to use reducing agents with high activity having secondary or tertiary alkyl groups (see JP-A-2001-209145), and to use development accelerators such as hydrazine compounds and vinyl compounds.
However, when these technologies were used, there was problematic in that density changes (printout property) with time after the thermal development processing became large and the silver color tone became extremely different (took on a yellow tinge) compared to wet type X-ray films in earlier technology. Additionally, a new problem where the color tone takes on a red tinge at high density areas with density of 2.0 or more has occurred when those with smaller average particle size are used as the silver halide.
JP Tokukai 2001-133925A discloses a technique for improving a printout property. Also, JP Tokukaihei-11-231460A, JP Tokukai-2002-169249A and JP Tokukaihei-11-288057A disclose technique for regulating silver color tone.